VISCOSITY CHANGES DURING MITOSIS 435 



movement and a direct measure of the viscosity. As shown 

 previously it requires on the average two turns of the centrifuge 

 handle (in two seconds) to produce zones in unfertilized eggs. 

 An arbitrary scale of viscosity may be established and the viscos- 

 ity of the protoplasm of unfertilized eggs can be taken as 2. At a 

 stage when sixteen turns are required to show zones the viscosity 

 in terms of the arbitrary unit is 16. 



Froni the data in the tables a viscosity curve may be plotted. 

 This is not, however, a simple matter. In general it is not pos- 

 sible to obtain direct readings of viscosity. Usually each test 

 indicates that the viscosity of the protoplasm is above or below 

 a certain value, but it does not show how much above or below. 

 Moreover, there is often some variation in the eggs of a given 

 lot, especially at times of transition. By noting for each test 

 whether the viscosity is above or below a certain value a curve 

 may be plotted by drawing a line above the one set of points 

 and below the other set. 



The following symbols were used in marking out points : 



V = Zones show plainly in all eggs. Viscosity above 



value at apex of V. 

 A =^ No zones appear in any eggs. Viscosity below value 



at apex of inverted V. 



Q = Zones appear but indistinctly (usually dark zone 



plainer than pigment zone). Viscosity about 



equal to value at center of circle. 



The above three symbols were combined in a variety of ways. 



X = Eggs with zones and without zones about equal 



in number. 

 ^ = Most eggs with zones, a few without. 

 A = Most eggs without zones, a few with. 

 In those cases where zones appeared indistinctly, the arc of 

 a circle was added to the upper or lower portion of any of the 

 last three symbols. 



In the curve the abscissa represents time in minutes after 

 fertilization. Of course for purposes of comparison the so- 

 called 'standard time' had to be employed. The ordinates 

 represent comparative viscosity values on the basis of the arbi- 

 trary scale mentioned above. 



